1. Field of the Invention
This invention relates to an energy subtraction processing method for radiation images, and a stimulable phosphor sheet, a stimulable phosphor sheet composite member, and a stimulable phosphor sheet-filter composite member which are used for the energy subtraction processing method. This invention particularly relates to an energy subtraction processing method, and a stimulable phosphor sheet, a stimulable phosphor sheet composite member, and a stimulable phosphor sheet-filter composite member used for the energy subtraction processing method in a radiation image recording and reproducing system wherein a stimulable phosphor sheet is once exposed to a radiation passing through an object to have a radiation image stored therein, the stimulable phosphor sheet is scanned with stimulating rays which cause it to emit light in proportion to the radiation energy stored, the emitted light is photoelectrically detected and converted to an electric image signal, and a visible image is reproduced by use of the obtained electric signal.
2. Description of the Prior Art
As one type of digital subtraction processing method or digital radiography (hereinafter referred to as "DR"), there has heretofore been known the energy subtraction processing method. In the energy subtraction processing method, an object is exposed to X-rays having energy distributions different from each other to obtain two X-ray images respectively containing the images of a specific structure (for example, an organ, bone, blood vessel, or the like) of the object recorded on the basis of the intrinsic X-ray energy absorption characteristics of the specific structure. Then, each X-ray image is weighted appropriately, and subjected to subtraction to extract the image of the specific structure. As regards DR, there has heretofore been known digital fluoroscopy wherein the output of an X-ray fluoroscopic camera comprising an image intensifier tube (I.I tube) and a television camera is digitally processed, and scanning projection radiography utilizing the X-ray detecting system of computed tomography, such as a Xe detector system.
Specifically, the following types of energy subtraction processing method have heretofore been known:
(I) An object is intermittently exposed at short time intervals to X-rays having energy distributions different from each other. In synchronization with the exposures, the X-rays passing through the object are detected by an X-ray fluoroscopic camera comprising an I.I tube and a television camera or by an X-ray detector such as a Xe detector. Then, a subtraction image is obtained from two or more X-ray images thus recorded. The exposures to the X-rays having energy distributions different from each other are effected by (i) modifying the X-ray source so that it can emit X-rays having such energy distributions different from each other, by (ii) closely positioning two or more X-ray sources emitting X-rays having energy distributions different from each other so that the X-rays emitted from the X-ray sources do not interfere with each other, or by (iii) selectively inserting and removing a filter for changing the energy distribution of X-rays into the space between the X-ray source and the object.
(II) Two X-ray sources capable of simultaneously emitting X-rays having energy distributions different from each other are positioned closely to each other. A filter made of an X-ray shielding material such as lead and provided with many fine slits or small holes (for example, small circular or square holes positioned in a checkered pattern) so that the area ratio of the X-ray shielding sections to the opening sections is 1:1 is inserted between the object and the X-ray sources in such a manner that the X-rays emitted from one X-ray source and the X-rays emitted from the other X-ray sources do not interfere with each other at the X-ray receiving face of the X-ray detector. In this condition, the object is simultaneously exposed to the X-rays emitted from the two X-ray sources, whereby X-ray images are formed on the X-ray receiving face of the X-ray detector by the X-rays having energy distributions different from each other. The X-ray images thus formed are detected by the X-ray detector, and discrimiated from each other when or after the X-ray images are read out. On the basis of the images thus discriminated, a subtraction image is obtained.
(III) An object is moved with respect to an X-ray source and an X-ray detector, and X-rays having energy distributions different from each other are alternately emitted at predetermined time intervals in a fan-like pattern. The X-rays passing through the object are detected by the X-ray detector positioned at the rear of the object. By use of the image signals thus obtained from the X-ray detector, X-ray images corresponding to the X-rays having energy distributions different from each other are obtained. Then, a subtraction image is obtained on the basis of the X-ray images. The X-rays having energy distributions different from each other may be generated in the same manner as described in (I).
(IV) A filter having the same construction as the filter used in the aforesaid method (II) is made of a metal absorbing the low energy component of X-rays, such as copper. The filter is inserted between an X-ray source and an object, and X-rays having energy distributions different from each other are generated from the X-ray source in such a manner that the X-rays do not interfere with each other at the X-ray receiving face of an X-ray detector. X-ray images are formed on the X-ray receiving face of the X-ray detector by the X-rays having energy distributions different from each other. The X-ray images thus formed are detected by the X-ray detector, and discriminated from each other when or after the X-ray images are read out. On the basis of the images thus discriminated, a subtraction image is obtained.
In the energy subtraction processing method, it is possible to discriminate and extract an image of a specific structure having X-ray energy absorption characteristics different from the characteristics of the other structures of an object, and to eliminate an image of the bone and form an image of only a soft tissue of the human body. For example, it is possible to discriminate and extract an image of a structure such as the bronchus existing at the mediastinal septum, which is superposed on the image of the bone and cannot easily be diagnosed in the conventional method, from the image of the bone. Further, when the so-called temporal (time difference) subtraction processing method is carried out in the recording of a contrasted image of the abdomen, a problem is presented by an artifact of the gas at the abdomen. However, in the energy subtraction processing method, it is possible to eliminate the information on the soft tissue and to form only an image of the bone and a contrasted image free from an artifact of the gas at the abdomen. Accordingly, the energy subtraction processing method can provide information useful for diagnosis which cannot be obtained by the conventional method, and is basically advantageous for medical diagnosis.
However, the aforesaid conventional energy subtraction processing method has drawbacks intrinsic to DR. Namely, the spatial resolution of the subtraction image obtained by use of the DR generally depends on the resolution of the X-ray fluoroscopic camera comprising an I.I. tube and a television camera, or the resolution of the X-ray detector such as a Xe detector. However, since the resolution of the X-ray fluoroscopic camera or the X-ray detector used in conventional DR is not so high, the conventional energy subtraction processing method presents the problem that it is impossible to diagnose a specific structure with sufficient accuracy. Further, since the recording range in DR is limited by the X-ray receiving area of the X-ray detector, the conventional energy subtraction processing method presents another problem in that it is impossible to obtain a subtraction image of a wide area of the human body at one time.
Furthermore, the aforesaid conventional energy subtraction processing methods (I), (II), (III) and (IV) have the drawbacks as described below.
1. A special X-ray source is needed. [Methods (I) and (II)].
2. A shift is generated between the corresponding picture elements of two X-ray images. [Methods (I), (II), (III) and (IV) wherein two X-ray sources are used].
3. The resolution obtained is half the resolution in the ordinary X-ray image forming method. [Methods (II) and (IV)].
4. Since the X-ray images obtained by use of the X-rays having energy distributions different from each other are formed on the same plane, it is not always possible to discriminate the X-ray images from each other when or after the X-ray images are read out. [Methods (II) and (IV)].
5. Since an object is intermittently exposed to the X-rays having energy distributions different from each other, a shift is generated between the images due to muscular motion, respiratory motion, vermicular motion, or the like, of the object. As a result, it becomes impossible to obtain a subtraction image of a high quality. [Method (I)].
6. Since an object is scanned with the X-rays emitted in a fan-like pattern, a relatively long time is required to form one image, and a time difference is generated between the beginning and the end of the scanning. Therefore, a shift is generated between the images due to muscular motion, respiratory motion, vermicular motion, or the like, of the object. As a result, it becomes impossible to obtain a subtraction image of a high quality. In particular this problem makes extraction of an angiogram all but impossible. [Method (III)].